Tip treatment for a rotor casing

ABSTRACT

A tip treatment bar and associated rotor casing structure, wherein the tip treatment bar includes a solid composite structure having a plurality of layers arranged therein so as to provide the bar with a directional stiffness characteristic. Such a layered composite arrangement can be used to tailor the vibration characteristics of the bar. The tip treatment bar and casing structure may be suited for mounting relative to a fan or compressor in a gas turbine engine.

The present invention relates to a casing for a rotor and more particularly to a casing structure comprising an array of bars. The present invention also relates to bars for a rotor casing.

Aerodynamic stall is a well known problem for bladed rotors, such as fans or compressors. The criticality of operation of such rotors typically determines the level of sophistication with which the onset of stall conditions is investigated and also the countermeasures put in place to avoid or mitigate the associated risks. Gas turbine engines represent an example of machines for which the aerodynamic performance of a fan or compressor is of critical importance to both performance and safety.

International Patent Application PCT/GB94/00481 (published as WO94/20759) describes a casing structure for a gas turbine engine compressor, which comprises an annular cavity located about the tip region of the compressor blades. Ribs are arranged about the cavity to define circumferentially spaced slots there-between. The effect of such a casing structure on the flow in the vicinity of the blade tips is such as to delay the onset of stall and, accordingly, such a structure is often referred to as an anti-stall tip treatment system.

WO 94/20759 is concerned almost entirely with the aerodynamic factors for delaying stall. However there are a number of other important factors which impact on the design of a suitable casing structure. Within gas turbine engine applications, for example, the consideration of weight, strength and fatigue all affect the possible implementation of such a casing structure.

Published UK Patent Applications GB 2 373 021, GB 2 363 167 and GB 2 362 432 each disclose advances to the basic system disclosed in WO 94/20759 for the purpose of accommodating such practical considerations. Those documents disclose various details of tip treatment bars and associated mounting arrangements with a view to reducing high cycle fatigue, whilst also allowing the bars to yield in the event that they are struck by a blade or blade fragment shed from the rotor.

However the inventors have identified that additional considerations, over and above those discussed in the prior art, can impact significantly on the function of such casing structures. For example, the geometry of the bar and/or associated casing structure can change during use, which can adversely affect the clearance between the blade tips and the casing, amongst other aerodynamic considerations. Furthermore the degree to which the damping can be controlled according to the embodiments of the prior art is not optimal.

Published UK Patent Application GB 2 373 024 discloses a coating that may be applied to a tip treatment bar in order to dissipate strain energy in use and thereby reduce the amplitude of vibrations. Whilst this does provide a partial solution to the problem of damping, it has been found that there is little scope to control the response of the bar during operation using such a bar construction.

Whilst any one of the above problems may be addressed in isolation, it is to be noted that any proposed solution to one such problem will also impact on another functional characteristic of the casing or rotor. Accordingly the above-described partial problems in fact relate to a mosaic of inter-related characteristics of the system which must be carefully balanced in order to arrive at an optimal solution.

It is an object of the present invention to provide a casing structure for a rotor which mitigates or satisfies the above-described problems to a greater degree.

According to the broad concept of the present invention, there is provided a tip treatment bar for a rotor casing, the tip treatment bar having a layered internal structure.

According to a first aspect of the invention, there is provided a tip treatment bar for a rotor casing, the tip treatment bar being of a substantially solid composite structure having a plurality of laminae or layers arranged therein so as to provide the bar with a directional stiffness characteristic.

The directional stiffness characteristic may be tailored to tune the bar such that the natural frequencies of the bar fall outside a normal operational frequency range of vibration, induced by the rotor.

The bar has a plurality of internal layers. The internal layers may be spaced from the outer surface of the bar by one or more external layers or coatings. A composite structure of this type may be considered to have a non-uniform core. Such a structure differs from a bar formed of a single uniform core material which has an outer coating layer applied thereto. The core of the composite structure may be considered to account for a central region which is anywhere between 50 and 95% of a cross sectional dimension or area of the bar. According to the invention, that core region may have a plurality of composite layers.

The aligned nature of the materials may be such that the composite structure may be generally uniform in a first direction, for example a longitudinal or axial direction, but non-uniform in a second direction, which may be perpendicular to the first direction, such as, for example, a transverse or radial direction.

The internal layers may be aligned with a longitudinal axis of the bar. Fibres within the at least one internal layer may be arranged obliquely to the longitudinal axis of the bar.

According to one particular embodiment, the materials within the bar may be arranged in a plurality of circumferential layers or laminae. One or more layers may extend across a width of the bar. The bar typically has opposing side walls, which may be substantially parallel, and one or more layers may be arranged obliquely to one or both of said side walls. One or more layers may pass through the core of the bar. The bar may have an outer layer comprising an erosion protection coating.

One or more of the internal layers within the bar may comprise a plurality of aligned fibres or wires. The fibres within one internal layer may be aligned differently or obliquely to fibres within a further composite internal layer. The central body or core material of the bar may itself be a composite material and may comprise a plurality of layers. Alternatively, the central body or core material may comprise a substantially solid uniform material or else a cellular material.

Typically the composite structure of the bar comprises first and second materials of different strengths and/or stiffnesses. The stiffer or stronger material may be considered to be a reinforcing material, whereas the less stiff or weaker material may be considered to be a matrix material. In one embodiment the composite structure comprises a resinous and/or polymer material, which may comprise a thermosetting plastic. The composite structure may be formed by a moulding process, such as, for example a resin transfer moulding (RTM) process.

The composite makeup of the tip treatment bar according to the present invention is advantageous since it allows improved damping characteristics when compared to the hollow or uniformly solid tip treatment bars of the prior art. The natural frequencies of the tip treatment bar can be tailored in a manner that was not hitherto possible. This is of particular benefit for rotor casing components since such components are typically exposed to a range of excitation during different operational conditions or states of the rotor. For example, within a gas turbine engine, the nature of the vibration will change between different aircraft flight phases or envelopes. However the fluid dynamics of rotor systems typically places tight geometrical constraints on components within the fluid flow, such as tip treatment bars. That is to say, the fluid-washed surface of such components must be first and foremost shaped for their aerodynamic purpose. Therefore the composite nature of the bar according to the invention provides a further degree of freedom for tailoring the vibration response of the bar, substantially without modifying its external geometry,

The natural damping achieved by an aligned composite layup may, in general, reduce the vibration amplitude across a range of excitation frequencies. In addition, the composite makeup may be tailored to avoid resonance in particular excitation zones which are known to be prevalent for particular types of rotor during operation.

In one embodiment of the invention, the layered composite structure may be arranged such that the natural frequency of the tip treatment bar lies outside the range of excitation frequency caused by the rotor during normal operation. Normal operation in respect of a gas turbine engine may comprise substantially steady-state or ‘cruise’ conditions. Additionally or alternatively, normal operation may comprise any or any combination of flight phases of an aircraft, such as take-off, climb and/or descent. Preferably harmonics may also be substantially avoided. Given the range of operating conditions of gas turbine engines, it is possible that a bar excitation frequency will cross an operational frequency of the rotor. However the present invention allows the bar to be tailored in such a manner that such excitation of the bar may be transient in manner.

The control of the vibration response of tip treatment bars in accordance with the invention can improve fatigue life. Furthermore the composite structure can be lightweight. An unforeseen benefit of the tip treatment bar composite structure for use in gas turbine engine applications is that the aligned composite structure beneficially accommodates the frangibility of the bar structure, particularly in a direction transverse to the composite material or laminae alignment. This frangibility is important in aiming to satisfy the response of the tip treatment bar to a ‘blade off’ scenario, such that the bar will yield upon impact with a blade or blade portion which is loosed at operational rotor speeds.

A yet further unforeseen benefit of the composite bar structure is that it can offer better control with regard to thermal expansion. Thus the geometry of the bar under thermal loading may be better predicted, which allows tighter control of blade tip clearance. This can have a knock-on benefit to the operation, for example, efficiency, of the rotor.

The composite structure may comprise multiple material layers or lamina. One or more layers may be entirely contained within an outer layer. A plurality of layers of the same or different material makeup may be generally concentrically arranged. The outer surface of the composite bar may comprise a plurality of layers. The bar may have an external layup as well as an internal layup. The bar may have a combination of internal and external layers.

One or more internal or coating layers of the bar may comprise a thermal barrier material.

According to a second aspect of the invention, there is provided a rotor casing comprising a plurality of bar members and a support structure arranged to maintain the bar members in a circumferentially spaced array with respect to an axis of rotation of the rotor in use, the bar members each having a composite structure.

Each bar member may comprise a substantially solid composite structure having a plurality of materials arranged therein so as to provide the bar with a directional stiffness characteristic. Each bar member may comprise a plurality of layers of different material therein. Each bar may be in accordance with the first aspect of the present invention and may comprise any of the optional features described in relation to the first aspect.

In one embodiment each bar is spaced or isolated from the support structure by a damping member. An individual damping member may be provided for each bar. The damping member may comprise a cuff member arranged about the bar and/or interposed between the bar and the support structure. The damping member may serve substantially to isolate the bar from the support structure.

The, or each, damping member may comprise a rubber material. The, or each, damping member may comprise a composite material arranged to provide the damping member with anisotropic stiffness characteristics. Damping means of this type can be tailored such that elastomeric damping members behave effectively in particular modeshapes using the anisotropic effect of the filler material. This can help further optimise natural frequencies of the bar and support structure when, for example, an unexpected mass is added to the bars such as coating for erosion protection.

The support structure may comprise one or more annular support members, which may be arranged about a rotational axis of the rotor. The support structure may comprise a plurality of openings, each arranged to receive a bar. The support structure may comprise a pair of spaced support members, such that the bars extend there-between. The openings in each support member may be shaped to receive an end of the bar members.

The directional stiffnesses of the bars and mounting system in combination may enable the amplitude of vibration to be controlled and/or the natural frequency of the tip treatment system as a whole to be tailored to substantially avoid resonance during rotor operation.

According to a further aspect of the invention, there is provided a gas turbine engine comprising a tip treatment bar according to the first aspect and/or a rotor casing according to the second aspect.

The term ‘directional stiffness’ as used herein may be considered to comprise, for example, axial, radial, torsional or through-thickness stiffness.

Practicable embodiments of the invention are described below in further detail by way of example with reference to the accompanying drawings, of which:

FIG. 1 is a partial axial sectional view of a 25 fan stage in a gas turbine engine;

FIG. 2 is a three-dimensional view of tip treatment bars mounted in a section of an annular support structure suitable for use in the engine of FIG. 1;

FIG. 3 is a three-dimensional view of a single isolated tip treatment bar and corresponding section of the support structure;

FIG. 4 is a cross section through the tip treatment bar of FIG. 3;

FIG. 5 is a three-dimensional view of damping members applied to a tip treatment bar; and,

FIG. 6 is a three-dimensional view of a further embodiment of a single isolated tip treatment bar.

FIG. 1 shows a fan casing 2 of a gas turbine engine. A fan, shown here by a single blade 4, is mounted for rotation in the casing 2. The fan takes the form of a fan blade assembly, having a plurality of assembled fan blades, which is mounted within the engine to a shaft, by which the fan is driven in rotation. Guide vanes 6 and 8 may be provided upstream and downstream, respectively, of the fan 4. The casing 2 includes a circumferentially extending chamber 10, which may be considered to comprise an annular void or recess.

The chamber 10 communicates with the main or global gas flow through the fan (represented by an arrow 12) through an array of slots 14 (see FIG. 2) defined between tip treatment bars 16 disposed around the casing. The function of the chamber 10 in delaying the onset of stalling of the blades 4 is disclosed in International Patent Publication W094/20759.

The tip treatment bars 16 are supported by annular end supports 18 to provide a tip treatment ring 20 (shown in FIG. 2), which is fitted within the casing 2 and extends around the fan 4. The bars are circumferentially spaced in a regular manner about the fan axis. The bars extend in a direction which is generally parallel with the rotational axis of the fan (i.e. they are spaced relative to the direction of travel of the rotor blade tips during operation), although they may be angled slightly relative thereto.

Vibration is induced in the bars 16 in operation of the engine at a frequency determined by the passage of the blades 4. This vibration can lead to fatigue failure of the bars 16. The vibrating bars 16 deflect in a generally circumferential direction as indicated diagrammatically in FIG. 2 by an arrow 21, and consequently fatigue failure tends to be initiated by cracking at the slot ends.

In the embodiments discussed below, the bars 16 are formed separately from the end support 18. The bars are generally quadrilateral in section and may take the form of a parallelogram. Such a profile is typically required to fulfill the aerodynamic requirements of the bars. The internal make-up of the bars is non-uniform as will be discussed below. More particularly, the internal make-up of each bar may be uniform in a first direction through the bar (for example, along its length) but may be non-uniform in a second direction, which is perpendicular to the first direction (for example, through a width or depth of the bar). This may otherwise be expressed as the bar makeup being non-uniform in section.

In the embodiment described below, the end supports 18 may be made from a carbon fibre/bismaleimide composite material, which enables the tip treatment ring to be light in weight while being capable of withstanding the is relatively high temperatures (in excess of 150 to 200° C.) encountered in operation. However alternative composite materials may be used, subject to the practical requirements of, for example, weight, strength and temperature resistance.

As can be seen in FIGS. 2, 3 and 6 the supports 18 comprise front 18 a and rear 18 b annular supports, which are located in respective upstream and downstream positions in use relative to the global flow direction 12 through the fan. The front and rear supports may be considered to constitute annular rails. Each of the front and rear supports has a plurality of openings 19, each opening being shaped to receive a corresponding end of a bar 16. The openings 19 are regularly spaced around the annular body of the supports. The openings 19 in the front and rear supports are aligned such that the bars extend there-between in regularly spaced array once assembled.

The openings generally take the form of radially extending slots so as to accommodate the shape of the bars, which, as shown in the embodiment of FIG. 2, have a depth dimension (in a radial direction in use) which is larger than the width of the bars (in a circumferential direction in use). The front and/or rear supports may have a radially inner 22 and/or outer 24 rim formation. Such rim formations may be oriented and/or dimensioned so as to allow for the desired degree of frangibility of the support members, for example in response to impact by a portion of a fan blade in use.

Turning now to FIG. 3, there is shown a general view of solid composite bar mounted within the openings 19 in the front 18 a and rear 18 b support rails. It can be seen that the bar comprises a solid composite structure in that it comprises a plurality of layers which make up the sectional area of the bar.

In this example, it can be seen in the sectional view of FIG. 4, that the bar 16 is formed of an inner body of material 26 surrounded by a plurality of circumferential layers 28-32 arranged around the inner body 26. Those layers are each of uniform depth along the length of the bar (i.e. in a direction between the opposing ends of the bar mounted in the front and rear supports respectively). The depth of each of those layers is also substantially uniform in a direction around the inner material 26.

In FIG. 4, it can also be seen that the section of the bar takes the form of a non-perpendicular parallelogram, such that the upper and lower surfaces of the bar are obliquely arranged or angled relative to the side walls. Accordingly, at least a portion of the internal layers, such as the transverse region of the layer that extends between the side walls, is also obliquely arranged relative to the side walls.

The inner body 26 is formed of a composite material, such as a fibre-reinforced material. The matrix material may be, for example, a maleimide such as bismaleimide (BMI) or an epoxy resin. The inner body is itself formed of a plurality of composite layers 27, which are generally parallel in alignment. The fibres in each of those layers are typically aligned in a common direction, which is shown as being oblique to the side walls of the bar. In alternative embodiments, the central body 27 could be formed of a solid, uniform or cellular material, such as a conventional polymer material.

Each of the layers 28 to 32 may comprise the same or a different material makeup to the inner body. Adjacent layers within the bar structure may also be different. However in this embodiment the layer 28 adjacent to the inner body 26 comprises substantially the same material makeup to the inner body. The composite layer 28 is oriented differently to the layers 27 in the inner body 26. The layer 28 is a circumferential layer which encloses the inner body 26. The fibres in layer 28 are also oriented differently to those of layers 27 and may be arranged, for example, substantially parallel with the side walls of the bar or else at an angle thereto which differs from that of the layers 27.

The options for each of the material layers 30 and/or 32 are the same as those described above for layer 28. Those layers may have the same or a different material makeup and/or fibre arrangement from layer 28 or body 26. For example, one or more layers may comprise the same composite material as the inner body, or else another layer, but it may be oriented such that the fibres therein lie in a generally different direction to the inner body or other layer.

In another embodiment, one or more internal layers 26-32 may comprise a non-composite material, such as a suitable polymer material. The materials used are subject to the preferred requirement that they are mouldable materials.

The outer layer 34 in this embodiment is a coating layer, which comprises a material suitable to provide an erosion protection coating. In this example, the outer layer material 34 comprises a fluoroelastomer, although any other suitably chemically and thermally stable polymer, or other, material could be used.

The inner body 26 and layers 28 to 32 may be considered to form a solid core of the bar. This construction has a plurality of internal layers and can be distinguished from the application of a coating layer to the exterior of the bar only. The ply of the internal composite layers may be of suitable thickness and may be in the region of 0.05 to 0.5 mm for example. In this example, a ply of approximately 0.1 mm was used. However the thickness of the different internal layers could also be different as necessary to optimize the bar for use.

As shown in FIGS. 3 and 4, a plurality of layers of the material may be exposed in one or more outer walls of the bar. In this embodiment, the outer material layer 34 of the bar does not completely surround the adjacent layer 32. In particular the outer layer covers three faces of the bar, shown as the three sides of the section view of the bar in FIG. 4. The adjacent layer 32 is thus exposed on one face of the bar. In this embodiment the layer 32 is exposed on a radially outwardly facing face of the bar when mounted for use in the front and rear supports 18. In this regard the bar can be described as having not only an internal composite layup but also an external layup, since a plurality of layers are exposed in the outer surface of the bar,

It has been found by the inventors that any or any combination of such internal and/or external layup can be used to significant benefit in providing the bar with a directional structural characteristic, such that the bar is orthotropic. Such directional properties are not only due to the fibre alignment of the inner body material but also due to the bar layup such that the bar can be considered to be ‘multiply orthotropic’. Having a solid tip treatment bar made from composite in this manner is advantageous as it provides directional stiffness which can be used to tune natural frequencies of the bar. Thus the material and layup can be tailored for a known vibration frequencies that occur during operation of the rotor to avoid zones of resonance during in service. Additionally or alternatively the amplitude of vibration of the bar itself can be reduced or minimized over a wide range of operational frequencies of the rotor (for example relating to different flight envelopes of an aircraft), thereby increasing the fatigue life of the tip treatment bar.

The damped response of a bar as described above is advantageous over and above the general inherent vibration damping benefits that occur due to use of a composite material per se.

Also it is of notable benefit that the above advantages can be achieved whilst maintaining a suitable frangibility of the bar (i.e. in a generally radial direction in use) to meet the fan blade containment requirements. This is due, at least in part, to the relatively brittle failure mechanism of composites compared to metals during impulse or impact events such as partial fan blade release. The containment design generally requires that a loosed fan blade part must be contained to avoid any hazardous events and that the blade fragment(s) are broken up through surrounding parts such as front and/or rear support rail and tip treatment bars.

The strength of any of the layers can be tailored as required, for example by improving inner and/or outer ply failure strengths.

A bar according to any aspect or embodiment of the invention may be formed using a moulding process. It has been determined that a wide variety of internal and external composite layup can be achieved using a resin transfer moulding process. This process moulds preforms, through resin injection, into the desired shape of the mould. The mould is in vacuum before injection and the pressure of injection wets the fibres in the mould. The nature of the preform defines the type of fibre in the mould which can hold its shape during the injection. Starting by bonding the inner body 26 as one or more infill blocks of the composite bar, layers 28 to 32 of the bar are wrapped and bonded around the infill blocks which can be located in the mould before it is closed. In this regard, the layers 28-32 can be co-formed or formed using a common moulding process. The inner body 26 may also be co-formed but in this embodiment it is preferable for practical reasons to form the layered inner body 26 separately.

The erosion coating 34 is bonded onto the bars, in this embodiment by a secondary bonding. Bismaleimide (BMI) or epoxy resins offer suitable thermal characteristics and low viscosity such that they can be resin transfer moulded. Resin Transfer Moulding is a cost effective method of manufacturing a solid multi-layer bar of organic matrix composites because it can be at least semi-automated. The coating 34 may be considered to provide a sheath about the core bar material.

A long length of the bar makeup can be produced using the methods described above and then cut into individual bars along its length.

In any or all of the above described embodiments, the bars are provided with damping members arranged interposed between the bars and the support structure 18. The damping members 36 are shown in FIG. 5 and comprise a damping material arranged around at least an end portion of the bar, typically at each end thereof. The damping material surrounds the outer periphery of the bar in the form of a collar or cuff formation, which may be referred to in the art as forming ‘damping boots’.

The damping members 36 space each end of each tip treatment bar 16 from the respective support 18. For this purpose, the openings 19 in each support 18 a or 18 b have generally the same shape as the cross-section of the tip treatment bars 16, but is substantially larger in dimension so as to accommodate the volume of the damping members therein. The space between each tip treatment bar 16 and the wall of the corresponding opening is filled by the damping material.

Each damping member may be formed as a separate component before assembly with its respective tip treatment bar 16 and the end supports 18. Alternatively, the boots may be formed by moulding the damping material in situ between the tip treatment bar 16 and the end support 18, in a potting process. The boots are bonded to the respective bars 16 and end supports 18 by means of 15 a-suitable adhesive, such as a silicone adhesive as is available under the name SILCOSET 152. The damping material itself could be a silicone elastomer, such as the material available under the name SILASTIC J.

By introducing a damping means in the form of an elastomeric intermediary between the bar and the front and rear rails, the natural frequencies of vibration of the vanes, when installed, are conventionally reduced. However the inventors have determined that this effect can be disadvantageous as the bar's natural frequencies of vibration may then be of such a value that they interact with the engine order forcing frequencies, which can result in greater bar vibration amplitudes. The inventors have determined that providing a relatively stiff material, such as, for example, aramid particles or layers or carbon nanotubes, inside the damping members can improve the performance of the system. Providing the damping member in the form of a composite material itself, with inherent stiffness and self damping properties, enables the mounting system stiffness to be modified in at least one direction without penalising damping characteristics.

The damping members may comprise an internal multi-layer composite.

Controlling the bonding between the damping members and the bar and/or support members is also an important consideration for controlling natural frequencies of the mounting system. The use of an adhesive would typically require the adhesive to be relatively compliant compared to the damping material itself and, as such, the adhesive can impact significantly the resultant stiffness calculation for the damping member arrangement. Even a relatively thin adhesive thickness, if applied over the entire bonding surface, will reduce the resultant stiffness considerably.

In view of the above, grooves may be provided in the inner and/or outer surfaces of the damping members for receiving an adhesive that bonds the damping member to the tip treatment bars and front/rear rails, in which the bar is mounted. Using such a grooved elastomeric damping member, the bonding can be achieved such that the adhesive is only present in the grooves of the elastomeric collar. In this regard the groove walls (i.e. protrusions) in the damping member surface can be formed so as to be an interference fit with the corresponding opposing surfaces of the bar and/or support member openings 19, which results in a squeezing effect on assembly, to urge the adhesive into the troughs of the grooves, rather than the outermost surface of the groove walls.

This configuration, and method of forming the assembly, has the effect that there is little or no adhesive at the outermost protruding portions of the damping member. As such, the mounting stiffness is dominated by the path between the bars and the front/rear support members via the damping material itself, such that the adhesive has very little effect on this load path. It has been found that, by varying the dimensions of the grooves, the stiffness of the mounting can be tuned to a desired frequency. In addition, if a specific modeshape can be excited by an up or downstream rotor/engine order, the natural frequencies could be simply reduced by increasing slightly glue line thickness and/or removal of support member or damping material.

Along with the above discussed methods of controlling the vibration response of the tip treatment bar assembly, such features can further provide for a system in which damping is enhanced. Accordingly a further definition of the invention may comprise a solid multi-layer composite bar configuration with enhanced damping members for mounting the tip treatment bar to a support member. Such enhanced damping members may comprise a stiffened composite material and/or grooves to allow improved control of adhesive dispersion between the damping member and bar or support.

Such an arrangement may provide a suitable thermal resistance and/or thermal mismatch between components to meet required system performance. The composite damping material and/or grooves may further facilitate the directional stiffness of the mounting system to improve design durability, for example by minimizing or avoiding zones of resonance during normal operation.

Turning now to FIG. 6, there is shown a further embodiment of the invention in which the geometry and internal layered structure of the bar has been modified. All other feature of those embodiments may be as described above and will not be repeated here for conciseness.

In the embodiment of FIG. 6, it can be seen that the bar 100 has an internal layer or body 102 which passes through the core of the bar. The layer 102 is sandwiched between further layers 104 and 106 on either side thereof. The layers 104 and 106 may comprise the side walls of a generally circumferential layer of the type described above.

In the embodiment of FIG. 6, the bar 100 has a central layer 102 which is generally parallel with the bar side walls and layers 104 and 106. The bar 100 is thus generally symmetrical about a mid plane thereof.

The bar 100 has upper 108 and lower 110 layers which extend between the bar side walls. The central layer 102 in this embodiment is an internal layer in that it is surrounded in section by the other layers 104 to 110. The layers 104 to 110 in this embodiment may be formed as a single circumferential layer. However the upper and lower edges of the bar have been cut away at each end of the bar so as to provide lateral recesses or grooves 112. One or more internal layers of the bar are thus exposed at those grooves 112 as shown. Modification of the outer shape of the bar in this manner is useful in mounting the bar, such that the damping members can be located or formed in the grooves 112. In this manner, the bar can be mounted in the support rails 18 such that the radially inner edge of the bar is substantially flush with the inner edge of the rotor casing. This helps to provide a minimum tip clearance between the bars and the fan blades 4. In this regard, the grooves 112 at the radially inner edge of the bar 100 may be deeper than the grooves 112 at the radially outer edge thereof to accommodate the thickness of both the damping members and also the casing structure.

The bar in FIG. 6 also has a coating layer 114 on a plurality of surfaces thereof, of the type described above. In this embodiment, the coating layer covers the side walls and a base wall of the bar 108 but not the radially outer wall, at which the layer 108 is exposed. Accordingly, such a layered structure has both an internal and external layup as described above. Any of the features of FIG. 6 may be substituted for any of the features described in relation to FIGS. 1 to 5 above wherever it is practicable to do so.

It is envisaged that further technical advantages may be achieved according to further embodiments of the invention by providing one or more of the bar layers with additional integral features as described below. Integrated features may include macro, meso, micro or nano scale damping material. For example, carbon nanotubes could be added into the reinforced structural resin system to further improve damping characteristic. Typically, approximately 1% nanotubes within the composite structure can increase structural damping by up to approximately 50% in a cost effective manner.

Also, different types of integrated features may be provided for different technical reasons. For example, the multi-layer makeup of the bar may accommodate one or more wire members arranged along the length of the bar between the front and rear support rails. Such wires may be arranged in, and aligned with, one or more of the layers. Such wires may offer greater tensile strength than the remainder of the bar, such that they work in conjunction with the frangibility of the bar to help fragment a blade or blade portion if it strikes the tip treatment bar assembly. That is to say, the wire(s) could act in the manner akin to so-called ‘cheese wire’ which serve to break up a blade portion upon impact therewith.

In another embodiment, one or more layers of the bar could accommodate optical fibres linking components in an engine instrumentation and/or engine health monitoring system. Such optical fibres could be embedded into the solid composite bar for engine test and service support.

In view of the above description, it is proposed that embodiments of the invention provide means to optimise natural frequencies of tip treatment bars, which is especially important for bars of the type shown in FIGS. 2, 3, and 6, the geometry and weight of which are highly constrained by aerodynamic performance requirements and the engine configuration. The response of the bars can be both tuned and damped in this manner. Also the support/mounting system for the bars can be designed to participate in the tuning of natural frequencies and overall damping by means of directional stiffness characteristics. Thus, the mechanical behavior of the bar and support system is such as to prevent critical primary bar mode excitation with specific engine orders for the running range.

Optional modifications to the above described embodiments are discussed below within the context of the invention.

In a further embodiment of the invention, one bar end can be fixed whilst the other end can be loosely constrained within a support member opening, for example to allow a constrained degree of motion in a direction which is parallel with the rotor axis. Alternatively, both ends can be flexible to allow the bars to move into the front and rear rail slots. Such options may be important to accommodate relatively high variation of coefficients of thermal expansion between components. The damping members may move into the support rail and flex to take into account thermal growth whilst solid composite bar configuration improves thermal resistance of tip treatment bars.

The tip clearance of the fan blade to the tip treatment bars is also required to be controlled for any engine transient or steady-state condition. The tip treatment bar and support system of the present invention has been found to allow suitable control of tip clearance in the normal operating temperature range of 160-200° C.

Any internal layer or coating of the tip treatment bars or support structure may comprise a thermal barrier material to enhance thermal resistance. The internal composite bar may comprise a high temperature capability resin system and/or a thermal barrier material, for example as an additional coating or layer or filler material therein.

Additionally or alternatively, the support members may comprise composite materials, for example, to improve frangibility. Any of the composite materials described above may provide damping enhancements by means of carbon nanotubes within the composite structure.

The invention can be as described above, where natural frequency reduction is enhanced by locally adding mass to any, or any combination of, the composite inner body or layers, the bar coating, bonding materials or the damping member.

The invention is particularly suited for tip treatment bars in a gas turbine engine. However the invention can be used in any rotor configuration where like or similar issues arise, such as within compressors, fans, pumps or the like. The invention is considered particularly beneficial when both a general level of damping is required and also where it is important to be able to control the natural frequencies of vibration of the mounted component. The invention may be suited for use with vanes at the low or intermediate pressure compressors in a gas turbine engine or other organic matrix composite components with similar requirements. 

1. A tip treatment bar for a rotor casing, the tip treatment bar having a composite structure comprising a plurality of internal layers arranged therein with each internal layer comprising a composite material having a plurality of fibres arranged within a matrix material, the bar having a longitudinal axis and fibres within at least one of said internal layer are substantially aligned in a direction which is angled to the longitudinal axis so as to provide the bar with a directional stiffness characteristic.
 2. A tip treatment bar according to claim 1, wherein the internal layers of the bar are spaced from the outer surface of the bar by a coating layer.
 3. A tip treatment bar according to claim 1, comprising first and second internal layers, wherein the fibres in a first layer are aligned in a direction which is different from the alignment of fibres in the second layer.
 4. A tip treatment bar according to claim 1, wherein the bar has a longitudinal axis and the internal layers extend substantially in the direction of said axis such that the layers are substantially uniform in a longitudinal direction and the bar is non-uniform in cross section.
 5. A tip treatment bar according to claim 1, comprising an inner body of material, wherein at least on internal layer is arranged circumferentially around said inner body.
 6. A tip treatment bar according to claim 5, comprising a plurality of layers arranged concentrically about said inner body.
 7. A tip treatment bar according to claim 5, wherein the inner body of material comprises a composite material having a plurality of layers therein.
 8. A tip treatment bar according to claim 1, comprising an outermost composite layer and a coating applied to a portion of said outermost composite layer such that a portion of the outermost composite layer is exposed on an external surface of the bar.
 9. A tip treatment bar according to claim 1, wherein the bar has a plurality of lateral edges extending between opposing sides of the bar and wherein the lateral edges are recessed with respect to the ends of the bar.
 10. A rotor casing comprising a plurality of bars according to claim 1 and a support structure arranged to maintain the bars in a circumferentially spaced array with respect to an axis of rotation of the rotor in use.
 11. A rotor casing according to claim 10, wherein each bar is spaced from the support structure by a damping member.
 12. A rotor casing according to claim 10, wherein the support structure comprises first and second annular support members arranged about a rotational axis of the rotor in use, each support member having an annular array of openings shaped to receive an end of each bar such that the bars are supported at their ends within the openings and extend between the first and second support members.
 13. A rotor casing according to claim 12, wherein the bars have a reduced depth dimension at each end thereof such that the portion of the bars extending between the support members has a radially inner surface that is substantially flush with an adjacent radially inner surface of the first and/or second support member when assembled for use.
 14. A rotor casing according to claim 10, wherein the support structure comprises a composite material.
 15. A gas turbine engine comprising a tip treatment bar according to claim
 1. 16. A tip treatment bar for a rotor casing, the tip treatment bar having a composite structure comprising a plurality of internal layers arranged therein with each internal layer comprising a composite material having a plurality of fibres arranged within a matrix material, wherein at least one internal layer is arranged circumferentially around said inner body so as to provide the bar with a directional stiffness characteristic.
 17. A tip treatment bar according to claim 1, wherein the internal layers of the bar are spaced from the outer surface of the bar by a coating layer.
 18. A tip treatment bar according to claim 16, comprising a plurality of layers arranged concentrically about said inner body.
 19. A rotor casing comprising a plurality of bars according to claim 16 and a support structure arranged to maintain the bars in a circumferentially spaced array with respect to an axis of rotation of the rotor in use.
 20. A rotor casing according to claim 19, wherein each bar is spaced from the support structure by a damping member. 